Mdadm is used in most Linux distributions to manage software raid where the server does not have or use a hardware raid controller This page will show some of the common commands and usages of mdadm. The bellow use RAID1 as an example, but can be modified for any raid level you are using.

Create a new raid array

mdadm --create --verbose /dev/md0 --level=1 /dev/sda1 /dev/sdb2


After we create our raid arrays we add them mdadm.conf depending on your OS this will be in /etc/mdadm.conf or /etc/mdadm/mdadm.conf (debian) we can add the array to the configuration file with the below.

mdadm --detail --scan >> /etc/mdadm.conf


mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Remove a disk from a raid array

You can’t remove a disk directly from an array unless it is failed, to remove an active (healthy drive) you first have to fail it before you can remove the drive.

Fail drive

mdadm --fail /dev/md0 /dev/sdb1

Remove drive

mdadm --remove /dev/md0 /dev/sdb1

Alternatively you can fail and remove the disk as a one liner

mdadm /dev/md0 --fail /dev/sdb1 --remove /dev/sdb1

Add a disk to an existing array

To add or replace a failed disk to an array

mdadm --add /dev/md0 /dev/sdb1

Check the status of the raid arrays

cat /proc/mdstat

Stop and delete a raid array

To remove a raid array you will have to stop it first and then remove it

mdadm --stop /dev/md0
mdadm --remove /dev/md0

To remove the superblock from the drives

mdadm --zero-superblock /dev/sdb

Replace and rebuild from failed disk in RAID-1

In this example the disk sdb has failed and been replace, to start the rebuild the disk partitions will need to be copied from the remaining working drive and then the raid set to rebuild.

With this command it will not prompt with any warning so make sure you type correctly as you may add more work for yourself.

sfdisk -d /dev/sda | sfdisk /dev/sdb

This will dump the partition table of sda and copy to sdb replacing all existing partitions. We now need to re add the partitions to the raid replace * with number i.e. sdb1 number depends on the output of cat /proc/mdstat you will need to do this for all partitions.

mdadm /dev/md0 --add /dev/sdb* 

You can then watch the status of the rebuild using watch

watch -n 10 cat /proc/mdstat

After the rebuild has completed you may need to reinstall grub if you have replaced a boot drive.

grub-install /dev/sdb

Hope this has helped you, remember to always double check before running commands.